I know it's hard to forgive someone who has hurt us; it's much easier to hold on to our hurts than to let them go. The memory of what they did and the damage we've suffered make it very difficult to forgive them.
This is why we need God's help, because only He can give us the strength we need to overcome our hurts and truly learn to forgive. And we need to do this, because otherwise we'll carry those hurts around with us the rest of our lives - and they will become like a massive, crushing burden. Long after the event has passed that caused our hurt, the anger and resentment and depression will still be with us - and (as I often say) they'll act like a poison in our minds and souls.
How can you avoid this? Begin by turning to God, admitting to Him that you know you can't do this on your own. Then ask Christ to come into your life and fill you with His love. We have treated God far worse than anyone has ever treated us - and yet He still loves us, and His Son was willing to die for us. The Bible says, "This is how we know what love is: Jesus Christ laid down his life for us" (1 John 3:16).
Then ask God to help you see other people - even your ex-husband - through His eyes. God hates the sin - but He still loves the sinner. The Bible says, "Forgive whatever grievances you may have against one another. Forgive as the Lord forgave you" (Colossians 3:13).